The continuous or partially batchwise preparation of MDA is, for example, described in EP 1 616 890 A1, U.S. Pat. No. 5,286,760, EP-A-451442 and WO-A-99/40059. The acidic condensation of aromatic amines and formaldehyde to form diamines and polyamines of the diphenylmethane series proceeds in a plurality of reaction steps.
In the aminal process, formaldehyde is firstly condensed with aniline in the absence of an acid catalyst to form aminal, with water being eliminated. The rearrangement to form MDA is then carried out in the presence of an acid catalyst in a first step to form para- and ortho-aminobenzylaniline. The aminobenzylanilines rearrange to form MDA in a second step. Main products of the acid-catalyzed reaction of aniline and formaldehyde are the diamine 4,4′-MDA, its positional isomers 2,4′-MDA and 2,2′-MDA and also higher homologues.
In the neutralization process, aniline and formaldehyde are converted directly in the presence of an acid catalyst into aminobenzylanilines which subsequently react further to form the two-ring MDA isomers and MDA homologues having more than two rings.
Regardless of the process variant for preparing the acidic reaction mixture, the work-up of the latter is commenced by neutralization by means of a base in the prior art. The neutralization is usually carried out at temperatures of, for example, from 90° C. to 100° C. without addition of further substances (H. J. Twitchett, Chem. Soc. Rev. 3(2), 223 (1974)). However, it can also be carried out at a different temperature level, for example to accelerate the degradation of interfering by-products. Hydroxides of the alkali and alkaline earth elements are suitable as bases. Preference is given to using aqueous NaOH.
After the neutralization, the organic phase is separated from the aqueous phase in a separation vessel. The organic phase containing crude MDA which remains after the aqueous phase has been separated off is subjected to further work-up steps, e.g. washing with water (base wash) in order to wash residual salts out of the crude MDA. The crude MDA which has been purified in this way is subsequently freed of excess aniline, water and other materials present in the mixture (e.g. further solvents) by suitable methods such as distillation, extraction or crystallization. The work-up customary in the prior art is disclosed, for example, in EP 1 652 835 A1, page 3, line 58 to page 4, line 13, or EP 2 103 595 A1, page 7, lines 21 to 37.
EP 1 616 890 A1 teaches that aniline and formaldehyde are firstly reacted in the absence of the acid catalyst to form aminal and the aminal is subsequently admixed with the acid catalyst and is reacted further at temperatures of from 20° C. to 100° C. and water contents of the acidic reaction mixture obtained in this way of from 0 to 20 percent by weight. In particular, the water is firstly at least partly removed from the aminal after the condensation of formaldehyde and aniline, with a water content of from 0 to 5 percent by weight being set in the aminal, and the aminal is subsequently admixed with acid catalyst and is reacted further at temperatures of from 20° C. to 100° C. and water contents of the acidic reaction mixture obtained in this way of from 0 to 20 percent by weight. Mixtures of the diamines and polyamines of the diphenylmethane series can in this way be produced at degrees of protonation of <15%, preferably from 4% to 14%, particularly preferably from 5% to 13%. Here, the degree of protonation is in the case of monoprotic acid catalysts (e.g. hydrochloric acid) the molar ratio of the amount of acid catalyst used and the molar amount of amine functions present in the reaction mixture. The patent application does not give any details concerning the procedure during shutdown of individual plant parts of an industrial production plant. The examples present therein are laboratory experiments. In particular, this patent application does not teach that merely individual plant parts and not necessarily the entire plant have to be completely rundown in order to effect shutdown.
The patent application EP 2 039 676 A1 is concerned with the preparation of MDA with the focus on neutralization and washing. It describes a mode of operation for optimizing the phase separation in the neutralization (step c) and/or washing (step e2). Part of the aqueous phase from the phase separation in the washing step e2) or the separation of aminal from the wastewater e3) is returned to the neutralization or washing. In other words: the configuration of an MDA plant in continuous normal operation is described. A procedure in the case of a shutdown of the MDA plant in which individual plant parts are taken out of operation and other plant parts are operated in the circulatory mode of operation is not disclosed in the patent application.
EP 0 283 757 A1 is likewise concerned with the preparation of MDA. The process described is characterized by the addition of aniline-free MDA's to aminobenzylamines formed by condensation of aniline and formaldehyde before they are rearranged in a reaction induced by heat. Example 2 describes a mode of operation in which a small part of the MDA formed is recirculated to the rearrangement reaction (cf. also claims 6 and 8). In other words: the configuration of an MDA plant in continuous normal operation is described. Details regarding the procedure when individual plant parts are shut down are not disclosed in the patent application.
WO-A-99/40059 teaches that in order to prepare methylenedi(phenylamine) by reaction of aniline with formaldehyde in the presence of acid catalysts in a semicontinuous process, aniline and optionally acid catalyst are initially charged, formaldehyde and optionally acid catalyst are fed through a mixing device into a circuit in which aniline, optionally acid catalyst and optionally previously introduced formaldehyde are circulated and the reaction mixture is brought to a temperature of greater than 75° C. after at least 50% of the total amount of formaldehyde to be introduced has been fed in. The introduction up to an amount of at least 50% of the total amount of formaldehyde to be introduced is carried out at a temperature of the reaction mixture in the circuit of from 20° C. to 75° C.
The quality of a process for preparing MDA is defined firstly by the content of undesirable by-products of the reaction in the product. Secondly, the quality of a process is defined by the overall process from start-up, normal production to running-down of the process being able to be operated without technical production failure or problems which require intervention in the process and by no losses of starting materials, intermediates or end product occurring.
Such problems can, for example, occur during start-up or running-down of the aminal reaction. Such problems can be, for example, high molecular weight solids being formed and leading to caking and blockages in the equipment (aminal vessel, aminal cooler and aminal separator and conduits). A further disadvantage is that all plant parts normally always have to be shut down when maintenance, repair and cleaning work is necessary on or in a reactor or another plant part since the process steps build on one another and are therefore always carried out in succession. As a result, the entire plant has to be emptied, which leads to a considerable amount of rejected material. Furthermore, energy has to be expended in order to bring reactors and plant parts back to the respective operating temperatures. Such production stoppages for plant inspections, repair measures and cleaning measures or shortage of raw materials or auxiliaries, planned or unplanned, are therefore always recurring plant states which have a considerable influence on the economical operation of a continuously operating plant or a continuously operating process.